There's a ton of unused art for this game, often with interesting history. I'll try to clear up a couple.

The early Molek is actually from the Haunted Cemetery. We reused, recolored, and reanimated the sprite for the Cultist Hideout. It started as a placeholder, but ended up fitting pretty well so we stuck with it. I assume he was for an early version of the Fate and Glory statue concepts, since he seems to have a stony look to him.

The "early Pumpkin Master" is also from the Haunted Cemetery. The Cem actually has an entire round that was unfinished, with a handful of enemies attacked to it. That one in particular is the Pumpkin King, presumably the intended boss for the round. Other enemies for the round are the Arena Spider, Arena Vampire, and Arena Risen Mummy. They all have basic behaviors and placeholder HP values, but they are functional. The round was probably scrapped for the sake of pacing. The dungeon is already long enough, it hardly needs a fifth round.

The Shatters boss sprites are the versions from the original idea document by LordShon. Those are just leftovers from before Kabam made their versions.

The Cupcake is indeed a Candyland enemy. It has the exact same behavior and attacks as the White Demon (surely a placeholder) but self destructs immediately due to the way its state transitions are set up.

The Parasite Chamber blob was an early version of the Swarm Colony. You can kind of see how that design evolved into the final sprite.

The bee sprite is a joke that Uni created. It's sometimes used as a placeholder.

Additionally, the large sheet with a ton of items was part of the Oryx Design Labs package that was used for most of RotMG's early sprites. Therefore, those (and many other sprites from the package) went unused.

Hopefully they'll make their debut someday when another endgame dungeon eventually comes along. That said, they would be in an awkward position due to their predecessors. Most T6 abilities are identical to their T5 equivalents except for a few meaningless stat bonuses. T6 abilities would have to be adjusted somehow before T7s come along or else the progression would be really wonky.

The problem is that you can't make abilities with temporary buff (cloak, helm) infinetely better due to the cooldown and the possibility to use that buff permanently. You can make T7 star or T7 shield (just make them do more damage), but you can't make T7 cloak in any way except stat bonuses.